Propellant compositions



..pellants.

United tates This invention relates to solid smokeless propellants useful for rocket jet propulsion and has for its object to improve and, increase the burning rates of such pro:

Generally propellants useful for this purpose comprise an intimate mixture of finely divided oxidizer, uniformly distributed in a resinous binder which acts as a fuel. The oxidizer is ordinarilydispersed in the fuel which is then polymerized to form a solid propellant grain. To avoid the formation of undesirable smoke, non-metal-containing ammonium and hydrazine salts are employed as oxidizers in propellants of this type.

y In the operation of jet motors it is customary to burn such solid propellant charges in the motor chambers to.

produce gas under pressure, which escapes at high velocity, generally through an exhaust nozzle leading from the chamber, thereby producing a resultant thrust. The combustion of a composite propellant is generally considered to be anexothermic reaction between gases evolved from the thermal decomposition of the oxidizer and the fuel. The intensity of the thrust produced is determind by the rate at which the propellant burns.

As the usefulness of a propellant is dependent upon the intensity of thrust created by its combustion and the consistent reproducibility of said thrust; it has long been the objective of those engaged in the art to find a means of increasing the burning rate while at the same time decreasing the effect of propellant temperature upon either or both the burning rate" and the equilibrium chamber pressure.

I have found that the presence of small amounts of ammonium trichromate, ammonium tetrachromate or a mixture thereof in a propellant composition results in increases of burning rates of as much as 45%, a reduction of as much as 50% in the temperature coefficient of burning rate at constant pressure and a reduction of as much as 2 5% in the temperature coefficient of chamber pressure at constant area ratio.

Ammonium trichromate and ammonium tetrachromate are thermally unstable and decompose almost instantly at temperatures in excess of 460 F. Although the nature of the reactioh'which the ammonium trichroinate and tetrachromate undergoes during the combustion of the propellant has not been conclusively proven, it has been hypothesized that the increase in the burning rate of the propellant is a result of the heat liberated by the decomposition of the polychromates as well as by their catalytic action. i

Ammonium. t-richromate and ammonium tetrachromate are elfect'ive forthbs'e purposes-in amounts of from about to about 4.0% by weight of the total propellant composition. 'The relative quantities of chromium being small iif'these'proportions, the amount of smoke formed during combustion is negligible.

-Sui table'oxidizers for" the smokeless propellants of the present invention are inorganic non-metallic salts, as for example, the nitrate, chlorate and "perchlorate salts of meats;

tent

The oxidizer should ordinarilybe supplied in suiiicient I fuel mixture should lie between 55% and 10%' of the I same propellant composition.

The fuel in which the oxidizer is dispersed is preferably a resin comprised of a polyester component, that is, a condensation product of a polybasic carboxylic acid and a polyhydric alcohol, in which there is incorporated an olefinic component.

The polyester component ordinarily possesses some degree of unsaturation in the molecule in order to permit polymerization with the olefinic component which may be for example; styrene, vinyl acetate, acrylic acid esters, methacrylic acid esters, allyl compounds such as allyl diglycol carbonate, diallyl maleate, diallyl glycollate, and other unsaturated components such as propylene butadiene, etc.; as well as derivatives of any of the above substances which are capable of polymerization with the resin. -In-general any olefin which will polymerize with the resin to form a solid grain may be employed; this includes all unsubstituted olefins and in addition many substituted olefins. u

The unsaturation present in the polyester permits the resulting unsaturated polyester to polymerize with the double bond in the vinyl, allyl, or other olefinic additives.

When a sufficient amount of cross linkage occurs the resin becomes thermosetting; with a' lesser degree of cross linkage the resin may be thermoplastic; and in some cases the resin possesses properties of both thermoplastic and thermosetting resins. All of 'these types of resins are Within the purview of the present invention.

The polyester component is prepared by conventional means, simply by condensing a polyhydric alcohol with a polycarboxylic acid; Mixtures may also be used. In

basic acids, including: oxalic, malonic, succinic, glutaric,

adipic, pimelic, sebacic, azelaic acids, etc., and the unsaturated carboxylic acids useful as the acidic components in forming polyester resins are maleic acid,-fumaric acid, citraconic acid, mesaconic acid, itaconic acid, etc. The anhydrides such as itaconic anhydride and phthalic anhydride, may likewise be used for supplying the desired unsaturation.

Regardless of'which of the saturated acids aretused, the degree of unsaturation necessary to provide cross linkage with the vinyl, allyl, or other olefinic components, may be obtained by the addition of any of the abovenamed unsaturated acids or their anhydrides.

The alcohols that can beused are not limited to the dihydric alcohols, as other polyhydric alcohols, such as the trihyd ric and higher polyhydric alcohols may be used. These aifordadditional possibilities for cross linking and as a consequence the toughness and brittleness of the final resin may be controlled as desired.

For the polyhydric alcohol component any of the fol lowingalcohols'may be used: dihydric alcohols, such as The amount of oxidizer added to the resinous ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol; a trihydric alcohol such as glycerol; tetrahydric alcohols such as theerythritols and pentaerythritols; pentitols which include arabitol, adonitol, xylitol; hexitols including mannitol, sorbitol, dulcitol; heptitols such as persitol and volamitol, etc., or mixtures of any of the above alcohols may be also employed if desired.

The condensation of the polyhydric alcohol and polycarboxylic acid is usually conducted in the presence of the monomeric vinyl, allyl or other olefinic component which forms cross linkages between the polymeric chains of the polyester. The amount of olefinic monomer may range from about 25% to about 100% by weight based on the weight of the monomer-resin mixture, and the amount of the monomer to be used in each case is determined by the particular properties which are desired in the finished resin. In general, 50% by weight of polyester to 50% by weight of the olefinic additive produces a satisfactory polyester type of matrix for the propellant. The olefinic monomers listed above are all liquids which dissolve in the alkyd resin forming mixtures having low viscosities and thereby facilitating the dispersion of the oxidizer throughout the liquid resin before curing.

The propellant is preferably compounded as follows: The oxidizer required to oxidize the fuel to the proper degree is uniformly mixed into the polyester resin-olefinic monomer mixture. A catalyst capable of accelerating polymerization of the resin mixture is added thereto and intimately incorporated therein before or at the time the oxidizer is added. In the same fashion the burning rate expediting catalyst of the present invention is added to the monomer in finely divided form. All of these substances including the oxidizer are thoroughly mixed with the liquid monomerand the resultant mixture is then cast into a suitable mold and the propellant substance cured.

The cast propellant is generally cured at temperatures ranging from ambient to about 220 F. When lower temperatures are employed the charge requires considerable time to cure. If shorter cure times are desired a more elevated temperature can be used. The time and temperature for curing makes no substantial difference in the ultimate product insofar as its ballistic properties are concerned.

Other components such as wetting agents to improve the castability of the composition may be added without departing from the scope of the invention.

. To illustrate the effect of the burning rate acceleration catalyst provided by the present invention upon propellants of the type herein described, 1% by weight ammonium trichromate was added to a propellant having the following representative formulation:

Percent Ammonium perchlorate 75.00 Polyester:

Diethylene glycol 47.00 Adipic acid 49.45 Maleic anhydride 3.55 Styrene l 12.35

Cumene hydroperoxide (polymerization catalyst) 0.25 Lecithin (wetting agent) 0.05

The burning rates at 60 F., 1000 p.s.i.a. and temperature sensitivity of said propellant with and without the burning rate catalyst are shown in'Table I.

To illustrate the efiect of varying amounts of the catalyst in the propellant a series of tests were made employing the above propellant formulation. The results of these tests are shown in Table II.

u and vr are temperature coeflicients defined as follows:

u, the temperature coefiicient of burning rate at constant chamber pressure, is usually calculated from burning conducted under controlled experimental conditions such as a Crawfordbomb in which constant pressure is maintained by the regulation of the inert atmosphere. It relates the rate of burning to temperature at a given pressure as:

percent change in burning rate at constant pressure 1 F. change in propellant temperature 1r the temperature coefficient of chamber pressure at constant area ratio, is usually calculated from burning conducted in a motor permitting varying pressure and is defined according tothe equation:

percent change in chamber pressure 1 F. change in propellant temperature 1r, is related to u by theapproximate relationship:

wherein n is the burning rate exponent, relating R, the burning rate, and p the pressure, by the equation:

R=cp 0 being a constant.

A reduction in the numerical values of these coefficients represents a decrease in the dependency of the burning rate on the temperature of the propellant. It can be seen from a comparison of the above data obtained from treated and untreated propellants that the addition of polychromate ammonium salts substantially increases the burning rate and decreases the dependency of burning rate and chamber pressure upon the temperature of the propellant.

In addition to the specific propellants described above the burning rates and ballistic properties of a wide variety of propellant formulations may be improved by the dispersion of polychromate salts of ammonia in the grains. These propellants have contained a variety of resins, representative examples of which are those comprising sebacic acid, maleic anhydride and propylene glycol in various molar ratios. These resins may be cross linked with olefins such as methacrylate, n-butyl acrylates, etc., and polymerized in the presence of various organic peroxides such as methyl ethyl ketone peroxide, cumene hydroperoxide. methyl amyl ketone peroxide. etc.

From the foregoing discussion it is evident that. b merely selecting an appropriate propellant composition, a polyester olefinic type fuel and an oxidizing ammonium salt, and incorporating into said propellant a polychromate ammonium salt, that improved ballistic properties and an increased burning rate may be induced.

It should be understood that the foregoing examples, although describing certain specific embodiments of, the invention, are provided primarily for the purpose Of illus tration and are not intended to impose any limitations upon its broader aspects.

9 A warps I claim:

1. A solid propellant composition comprising from about 45% to about 90% by weight of the total propellant composition of a non-metallic, inorganic oxidizing salt, and from about 55% to about 10% by weight of an unsaturated polyester resin consisting of the condensation product of saturated polyhydric alcohol and polycarboxylic acid heteropolymerized with an unsaturated compound selected from the group consisting of lower alkenes, lower alkynes, phenyl substituted lower alkenes, lower alkyl dienes, lower alkenyl esters of lower alkenoic acids, lower alkenyl esters of lower alkanoic acids, lower alkyl esters of lower alkenoic acids, allyl diglycol carbonate, diallyl diglycollate and mixtures thereof; and a burning rate acceleration catalyst selected from the group consisting of ammonium trichromate, ammonium tetrachromate and mixtures thereof, in an amount of from about 0.10% to about 4.0% by weight of the total propellant composition. I

2. A solid propellant composition comprising from about 45% to about 90% by weight of the total propellant composition of a non-metallic, inorganic oxidizing salt selected from the group consisting of ammonium nitrate, ammonium perchlorate, ammonium chlorate and mixtures thereof, and from about 55 to about 10% by weight of an unsaturated polyester resin consisting of the condensation product of saturated polyhydric alcohol and polycarboxylic acid heteropolymerized with an unsaturated compound selected from the group consisting of lower alkenes, lower alkynes, phenyl substituted lower alkenes, lower alkyl dienes, lower alkenyl esters of lower alkenoic acids, lower alkenyl esters of lower alkanoic acids, lower alkyl esters of loweralkenoic acids, allyl diglycol carbonate, diallyl diglycollate and mixtures thereof; and a burning rate acceleration catalyst selected from the group consisting of ammonium trichromate, ammonium tetrachromate and mixtures thereof, in an 6 amount of from about 0.10% to about 4.0% by weight of the total propellant composition.

3. A solid propellant composition comprising a cured intimate mixture of from about to about 90% by Weight of the total propellant composition of ammonium perchlorate, and from about to about 10% by weight of an unsaturated polyester resin consisting of the condensation product of diethylene glycol, adipic acid and maleic anhydride heteropolymerized with a mixture of styrene and methyl acrylate; and ammonium trichromate in an amount of from about 0.10% to about 4.0% by Weight of the composition.

4. A solid propellant composition comprising a cured intimate mixture of from about 45 to about by weight of the total propellant composition of ammonium perchlorate, and from about 55% to about 10% by weight of an unsaturated polyester resin consisting of the condensation product of diethylene glycol, adipic acid and maleic anhydride heteropolymerized with a mixture of styrene and methyl acrylate; and ammonium tetrachromate in an amount of from about 0.10% to about 4.0% by weight of the composition.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Hackhs Chemical Dictionary, 3rd edit., The Blakiston Co., Phila., Pa., page 593. (Copy in Scientific Library.) 

1. A SOLID PROPELLANT COMPOSITION COMPRISING FROM ABOUT 45% TO ABOUT 90% BY WEIGHT OF THE TOTAL PROPELLANT COMPOSITION OF A NON-METALLIC, INORGANIC OXIDIZING SALT, AND FROM ABOUT 55% TO ABOUT 10% BY WEIGHT OF AN UNSATURATED POLYESTER RESIN CONSISTING OF THE CONDENSATION PRODUCT OF SATURATED POLYYDRIC ALCOHOL AND POLYCARBOXYLIC ACID HETEROPOLYMERIZED WITH AN UNSATURATED COMPOUND SELECTED FROM THE GROUP CONSISTING OF LOWER ALKENES, LOWER ALKYNES, PHENYL SUBSTITUTED LOWER ALKENES LOWER ALKYL DIENES, LOWER ALKENYL ESTERS OF LOWER ALKENOIC ACIDS, LOWER ALKENYL ESTERS OF LOWER ALKANOIC ACIDS, LOWER ALKYL ESTERS OF LOWER ALKENOIC ACIDS, ALLYL DIGLYCOL CARBONATE, DIALLYL DIGLYCOLLATE AND MIXTURES THEREOF, AND A BURNING RATE ACCERLATION CATALYST SELECTED FROM THE GROUP CONSISTING OF AMMONIUM TRICHROMATE, AMMOMIUM TETRACHROMATE AND MIXTURES THEREOF, IN AN AMOUNT OF FROM ABOUT 0.10% TO ABOUT 4.0% BY WEIGHT OF THE TOTAL PROPELLANT COMPOSITION. 